Liver fibrosis and cirrhosis are the end result of chronic liver disease from multiple causes, affecting 400,000 people and resulting in more than 20,000 deaths in the United States annually. Fibrosis occurs at the cellular level when a variety of cell types activate to fibrogenic, alpha-smooth muscle actin (alpha-SMA)- expressing myofibroblasts. In the liver, hepatic stellate cells (HSC) and portal fibroblasts (PF) are considered the two most important cell types to undergo activation. Factors mediating the activation of HSC and PF are poorly understood in spite of a decade of intensive study. We have studied the process of HSC activation in vitro, and have proposed a preliminary two-step model in which the myofibroblast phenotype results from an interplay between mechanical and soluble factors. In this model, HSC express alpha-SMA when subjected to mechanical tension from the extracellular matrix (ECM) and then, under the influence of TGF-beta signaling via the cytoplasmic signaling intermediate SmadS, organize the alpha-SMA into stress fibers, becoming fully activated myofibroblasts. The goal of this proposal is to develop a detailed understanding of the role of mechanical factors (adhesion-dependent mechanical tension) in hepatic myofibroblast activation in vitro and in vivo. Our hypothesis is that the mechanical properties of the ECM in the context of TGF-beta signaling are critical for the activation of HSC and PF to myofibroblasts in fibrosis. We propose to address this goal and hypothesis through three specific aims: 1) To determine the role of integrins and downstream signal transduction pathways in mechanosensing in HSC activation in vitro;2) To determine the role of matrix stiffness in PF activation and to develop a mechanical model of the liver that incorporates HSC and PF activation;3) To identify the mediators of matrix stiffness in vivo and to determine their role in fibrosis. The proposed studies will provide important new information about the factors involved in myofibroblast transdifferentiation in liver fibrosis, offering the potential for significant advancement in understanding and treating fibrosis, particularly in the identification of new factors regulating early fibrosis.